1. Field of the Invention
The present invention relates to a wire electric discharge machine with a wire electrode cutting mechanism.
2. Description of the Related Art
In wire electric discharge machining, unexpected breakage of an electrode wire may intermittently occur, depending on the complexity of the machining shape, machining conditions, physical properties of the material of a workpiece, etc. To overcome the breakage problem during the machining, a wire electric discharge machine is provided with a wire-breakage repairing program. Wire breakage repairing is performed and completed based on processes previously set in the wire-breakage repairing program. These sequential processes include occurrence of breakage during machining, rewinding of the electrode wire, return of upper and lower guides to their respective machining start points based on control of drive axes, cutting of the distal end of the electrode wire, wire connection, completion of the wire connection, return of the upper and lower guides to a breakage position, and restart of machining.
Japanese Patent Application Laid-Open No. 8-309622 discloses a technique in which the distal end portion of a wire on the wire supply side is partially cut and removed if wire breakage occurs in an electric discharge machining section during wire electric discharge machining. The wire is automatically connected in the position of a machining start hole and retraced to a wire breakage position, and electric discharge machining is restarted at the position where the wire is broken.
A prior art wire electrode cutting mechanism will be described with reference to FIG. 6.
In a wire-breakage repairing mode where wire breakage caused during machining is repaired, the drive axes of the wire electrode cutting mechanism are controlled so that the upper and lower guides are relatively restored to the machining start points. Thereafter, an electrode wire 1 is straightened by annealing and its distal end is cut, whereupon wire connection is started. A stress, such as abnormal electric discharge that suddenly occurs between a workpiece and the electrode wire 1, may be one of the main causes of breakage of the wire 1. Since sudden abnormal electric discharge produces high local current, it fractures the electrode wire 1 at a stroke.
The electrode wire 1 being machined is continually subjected to a tension adjusted under the machining conditions. If a sudden fracture occurs, therefore, the tension is instantaneously released, so that the electrode wire 1 is inevitably bent like a spring by reaction force. In wire breakage repairing during taper machining, in particular, the electrode wire 1 is rewound through a tapered machining groove, so that its distal end may sometimes be severely bent by contact with the workpiece as it is taken up.
In a cutting/connection mode that is activated on completion of the return of the upper and lower guides to the machining start points after the electrode wire 1 is rewound, a brake roller 6 is rotated in a wire delivery direction (indicated by arrow 101 in FIG. 6) to deliver the electrode wire 1 to a detection-unit electrode 5, in order to straighten the electrode wire 1 taken up by an AWF (auto wire feeder) pipe 4 by annealing and cut the damaged distal end of the wire.
Thereafter, cutting-start annealing torque is produced by the brake roller 6 to straighten the electrode wire 1 as the wire is cut at the position of an AWF pipe nozzle 9 near the detection-unit electrode 5 by energizing the electrode 5 and a chuck-section cutting electrode 7. To attain this, the distal end portion of the electrode wire 1 to be cut should be clamped by a pickup unit 8.
As mentioned before, the distal end of the electrode wire 1 is often extremely bent at the time of wire breakage repairing. If the degree of bending is high, the distal end of the delivered electrode wire 1 may be derailed to the outside of the pickup unit 8, failing to be inserted into the frame of the pickup unit 8 (see an extremely bent end portion 100 of the wire indicated by a broken line in FIG. 6). The electrode wire 1 having failed to be clamped by the pickup unit 8 due to derailing cannot obtain a supporting point for clamping, so that it cannot receive either cutting-start annealing torque produced by the brake roller 6 or cutting current. Inevitably, therefore, cutting failure occurs, thereby stopping the entire wire electric discharge machine.
As described above, the derailing of the electrode wire 1 implies a state in which the distal end of the electrode wire 1 is not clamped by the pickup unit 8 at the time of wire breakage repairing. According to this prior art technique, a distance 103 between the AWF pipe nozzle 9 and the pickup unit 8 is long, as indicated by arrow 102 (wire rewinding direction).